164 



THE RISE OF ANIMAL LIFE 



brain 



Fig. 9-4. Planaria in dorsal view and cross-section, 

 showing the excretory system on the left and the 

 nervous system on the right. Upper left indicates side 

 view of how cut was made. 



of the coelenterates. Perhaps most unique is 

 the concentration of the nervous tissue in 

 the head region below the eyes. The nerve 

 cell bodies are contained in two masses of 

 nervous tissue, the cerebral ganglion, com- 

 monly referred to as the brain. From this 

 concentrated point two longitudinal nerve 

 cords pass posteriorly and two short nerves 

 extend anteriorly to connect with the eyes. 



Along the two longitudinal cords are many 

 transverse nerves, which are distributed to 

 the internal structures of the body. 



The eyes of planaria are found on the 

 dorsal surface where they appear as two 

 dark spots (Fig. 9-5). There is no lens, as 

 such, although the ectoderm over the eye 

 is without pigment so that light can pass 

 through to reach the sensory cells below, 

 which connect with the brain. Without a 

 lens no image is possible, but the eye is 

 sensitive to varying intensities of light and 

 the animal withdraws from bright light and 

 seeks out moderate illumination. Other 

 sensory cells protrude from the surface of 

 the body and act as receptors for register- 

 ing changes in the flow of the water or 

 other variations in the surroundings. 



Although it is evident that the nervous 

 system of the Turbellaria is still very sim- 

 ple, the increase of special sensory cells, 

 their grouping into such an organ as the 

 simple eye-spot, and the aggregation of 

 nerve cells into the cerebral ganglion, are 

 the beginnings of a definite central nervous 

 system. 



Fig. 9-5. Head of planaria {Euplanaria or 

 highly magnified. Note the "crossed" eyes 

 two ear-like extensions of the head region 

 tactile sense organs are located. 



Dugesia) 

 and the 

 in which 



